In optical signal transmission techniques in the past, there has been a WDM (wavelength division multiplex) procedure as a communication procedure of multiplexing a plurality of optical signals having different wavelengths into an optical fiber cable. In the WDM procedure, an optical transmission device can send the optical signals to a sending destination different for each wavelength. In a case of carrying out amplification of an optical output level, the optical transmission device that relays the optical signals sometimes have different numbers of relays or transmission distances depending on the routes of the optical signals. In this case, variations (hereinafter, referred to as “a tilt”) in the optical output level turn out to occur between the wavelengths. As a result, a tilt sometimes does not fall within a receivable range of the output levels of optical signals (dynamic range) in an optical transmission device on the receiving side. With that, in order to reduce the tilt, the optical transmission device monitors the output level for each wavelength when relaying an optical signal and feeds back the results to the signal control to provide for equalization of the optical output levels (see Japanese Laid-open Patent Publications No. 2010-81398, No. 10-163960, and No. 9-51323).